Electrolytic method for refining titanium metal



Feb. 14, 1956 F. J. SCHULTZ ET AL ELECTROLYTIC METHOD FOR REFININGTITANIUM METAL Filed Oct. 18, 1951 INVENTORS Frank J. Schultz BY ThomasM. Buck RNEY United States PatentO ELECTROLYTIC METHOD FOR REFININGTITANIUM METAL Frank J. Schultz, Fords, and Thomas M. Buck, Plainfield,N. J., assignors to National Lead Company, New York, N. Y., acorporation of New Jersey Application October 18, 1951, Serial No.251,902 3 Claims. (Cl. 204-64) It has been found that it is difiicult torefine elcctrolytically impure titanium metal since titanium ismulti-valent and, therefore, forms a number of valence states in theelectrolytic medium used for such a refining process.

For example, it is possible to formquadrivalent, trivalent,

and divalent compounds such as titanium tetrachloride, titaniumtrichloride, and titanium dichloride. Furthermore, an aqueous solutionmay not be used as the electrolytic medium because reduced titaniumsalts may react with water. With respect to using molten saltelectrolytes, it has been found that reduced titanium compounds such astitanium dichloride and titanium trichloride are soluble whereastitanium tetrachloride is not. With such a variety of titanium compoundshaving different valence states, i. e. titanium tetrachloride,trichloride and dichloride, it is diilicult to refine impure titaniummetal by such a process.

An object of this invention, therefore, is to provide a method forrefining impure or crude titanium metal by an electrolytic process whichis simple and economical to operate. A still further object of theinstant invention is to provide an electrolytic refining process for theproduction of titanium metal which produces substantially pure andductile titanium metal. These and other objects will become apparentfrom the following more complete description of the instant invention.

Broadly this invention contemplates a method for refining titanium metalwhich comprises passing electric current through an electrolytic cellhaving a cathode, and a molten halide salt electrolyte containingreduced titanium chlorides and employing impure titanium metal as theanode. By the term reduced titanium chlorides we mean titaniumdichloride and titanium trichloride. The titanium metal employed as theanode is composed of the impure or crude titanium metal to be refined.The fused salt electrolyte should contain a concentration of from 0.1 to5.0 molal reduced titanium chlorides of which at least 40% is titaniumdichloride. The refining process should be carried out by maintainingduring the passage of said current an anode current density from 0.01 to3.0 amperes per square centimeter, and a cathode current density of atleast 0.1 ampere per square centimeter but not exceeding five times thenumerical molality of the reduced titanium chlorides present in theelectrolyte.

For refining impure titanium metal to produce ductile titanium metal ofexceptional quality, it is desirable to have at least 70% of the reducedtitanium chlorides in the electrolyte present as titanium dichloride,the re 2,734,856 Patented Feb. 14, 1956 mainder being titaniumtrichloride. Furthermore it is preferred in such instances to havepresent a concentration in said electrolyte of from 0.3 to 2.0 molalreduced titanium chlorides. The anode current density preferably shouldbe from 0.1 to 0.3 amperes per square centimeter and the cathode currentdensity from 0.2 to 1.5 amperes per square centimeter during the passageof the current through the cell.

Fig. 1 is presented to illustrate more clearly in cross section the typeof electrolytic cell which has been successfully used in refiningtitanium metal according to the instant invention. Referring to Fig. l,the electrolytic cell consists of container 11 which is heatedexternally by gas flames 12 in furnace 13. The halide salt electrolyte14 is placed in the container. A cathode 15 is inserted into the celland extends down into the electrolyte. The

cathode may be composed of any suitable material such as nickel. Theanode 16 consists of impure titanium metal which is to be refined. Thecell is further provided with vent 17 to permit the removal of anygaseous products.

The particular type of anode is unimportant and substantially anysintered, pressed or solid form may be used. It is particularlydesirable to consolidate as by sintering or pressing finely dividedimpure titanium metal to be refined into the form of a compacted mass,for instance rods, sheets, bars and the like. These compacted masses arethen immersed into the fused salt bath as the anode. However, many othermethods may be used to prepare the anode for example, suspending theimpure titanium metal in the fused salt bath by providing a containersuch as a metallic basket, e. g. one constructed of nickel or othersuitable material in which the impure titanium metal may be placed. Fig.2 shows pieces of impure titanium metal 18 in such a metallic anodebasket 16A.

The .halide salt electrolyte preferably comprises a molten halidemixture of alkali or alkaline earth metals or magnesium. It isparticularly desirable to employ the chlorides of "said metals becauseof the economics involved. These halides may be employed singly or incombinations. Mixtures of these halides which form low melting pointeutetics are most. convenient to em ploy.

The halide salt electrolyte should contain reduced titanium chlorides inconcentration from 0.1 to 5.0 molal, of which at least 40% is titaniumdichloride. In order to make ductile metal orexceptional quality thehalide salt electrolyte should contain reduced titanium chlorides inconcentration from 0.1 to 5.0 molal, preferably from 0.3 to 2.0 molal,the reduced titanium chlorides being at least titanium dichloride. Ifthe percentage of titanium dichloride is less than 40%, a titanium metalproduct is obtained which is no purer than the starting titanium metaland hence no refining is performed.

An atmosphere of argon or other inert gas is preferably maintained abovethe molten salt bath in order to prevent contamination of the titaniumvalues by the outside atmosphere.

In order to carry out the process of the instant invention the impuretitanium metal which acts as the anode is introduced into the fused saltbath which contains reduced titanium chlorides. An electric current isthen passed through the cell. The impure titanium metal at the anode isthen converted to solubilized titanium chlorides which are transferredthrough the salt bath bottom of the'cell. The titaniummetal at the anode3 therefore is solubilized, transferred and subsequently deposited onthe cathode in a purified state.

In operating the cell it is necessary to maintain current densitieswhich fall within certain limits in order to refine impure titaniummetal. It has been found that it is necessary to maintain an anodecurrent density from 0.01 to 3.0 amperes per square centimeter and acathode current density of at least 0.1 ampere per square centimeter butnot in excess of five times the numerical molality of the reducedtitanium chloride present in the electrolyte. If the anode currentdensity is less than 0.01 ampere per square centimeter, the rate oftitanium metal refined is excessively slow and therefore the throughputefliciencies are excessively low. 'If an anode current density exceeds3.0 amperes per square centimeter, there is too much opportunity fortitanium dichloride present in the salt electrolyte to be converted totitanium trichloride which results in the titanium values beingchlorinated instead of producing titanium metal. The current densitiesof the electrodes are dependent on both the current passing through thecell and the effective electrode area, that is that portion of thesurface area of the electrode at which electrolysis actually occurs. Ifthe cathode current density is less than 0.1 ampere per squarecentimeter, titanium metal will not be deposited. If the maximum currentdensity is exceeded, metals other than titanium, that is, those from thefused salt electrolyte will be deposited.

For ductile titanium metal of exceptional purity the anode currentdensity should be from 0.1 to 0.3 ampere per square centimeter and thecathode current density from 0.2 to 1.5 amperes per square centimeter.

In order to describe more fully the invention, the following examplesare presented:

EXAMPLE 1 An electrolytic cell of the type shown in Fig. 1 was used torefine impure titanium metal. A chloride salt electrolyte consisting of7300 grams of strontium chloride and 2700 grams of sodium chloride wasintroduced into the cell and heated at a temperature of 700 C.

The electrolyte was charged with 1425 grams titanium dichloride and 463grams titanium trichloride equivalent to a concentration ofapproximately 1.5 molal reduced titanium chlorides. About 75% of thetotal reduced titanium chlorides was titanium dichloride and theremainder titanium trichloride. The impure titanium metal which was tobe refined was compressed into the form of a solid cylindrical anode bysintering and the impure titanium metal anode was inserted into theelectrolyte. The impure titanium metal which had been prepared by theprocess described in U. S. Patent No. 2,205,854 had a Brinell hardnessnumber of 361. A spectrographic analysis of the impure metal is reportedin Table I.

An electric current of 65 amperes at an impressed voltage of 1.6 voltswas then passed through the electrolytic cell. The cell was run with acathode current density of 1.0 ampere per square centimeter and an anodecurrent density of 0.25 ampere per square centimeter. The cellresistance was approximately 0.02 ohm.

The impure titanium metal at the anode was then converted to solubilizedreduced titanium chlorides which were transferred through the fusedsaltelectrolyte to the cathode at which point they were electrolyticallyreduced to titanium metal which was deposited on the cathode. Theimpurities which were present in the titanium metal forming the anodesettled out as sludge material in the bottom of the cell.

The purified titanium metal which had deposited at the cathode wasremoved from the fused salt bath and leached with dilute hydrochloricacid. The metal was then washed with distilled water and dried at -60 C.The purified metal was then melted in an arc furnace to produce ametallic mass which had a Brinellhardness number of 204 and which wasmuch purer than 4 the impure starting material as indicated by thespectrographic analysis also recorded in Table I.

EXAMPLE it Another sample of impure titanium metal having a Brinellhardness number of 420 was refined electrolytically in the manneridentical to that described in Example I. Again the impure titaniummetal was sintered into the form of a cylindrical solid and the impuretitanium metal anode inserted into the fused salt electrolyte. Theoperating conditions for this refining process were identical to thosedescribed in the previous example. The purified titanium metal whichdeposited at the cathode in this experiment was also melted into ametallic mass and had a Brinell hardness number of 212. The impurestarting metal and the purified metal deposited at the cathode wereanalyzed spectrographically as reported in Table I.

Table l SPECTROGRAPHIC DATA It has been shown by the description and theexamples presented above that impure titanium metal may be refined bypassing electric current through an electrolytic cell in which theimpure titanium metal acts as the anode and in which the molten saltelectrolyte contains reduced titanium compounds. Furthermore it has beenestablished that high purity ductile'titanium metal may be produced byan electrolytic refining process which is simple and economical tooperate.

While this invention has been described and illustrated by the examplesshown, it is not intended to be strictly limited thereto and othermodifications and variations may be employed within the scope of thefollowing claims.

We claim:

1. Method for refining titanium metal which comprises passing electriccurrent through an electrolytic cell having an anode and a cathode and amolten halide salt electrolyte, employing impure titanium metal as theanode in said cell, said electrolyte being composed of at least onecompound selected from the group consisting of the halide salts ofalkali metals, alkaline earth metals, and magnesium, said electrolytefurther containing soluble titanium compounds selected from the groupconsisting of titanium dichloride and titanium trichloride inconcentration from 0.1 to 5.0 molal, said soluble titanium compoundscomprising at least 40% titanium dichloride, and maintaining during thepassage of said current an anode current density from 0.01 to 3.0 ampereper square centimeter, and a cathode current density in amperes persquare centimeter ranging from 0.1 up to 5 times the numerical molalityof the soluble titanium compounds.

2. Method for refining titanium metal which comprises passing anelectric current through an electrolytic cell having an anode, cathodeand a molten halide salt electrolyte, employing impure titanium metal asthe anode in said cell, said electrolyte composed of at least onecompound selected from the group consisting of the halide salts ofalkali metals, alkaline earth metals, and magnesium, said electrolytecontaining soluble titanium compounds selected from the group consistingof titanium dichloride and titanium trichloride in concentration from0:3 to 2.0 molal, and maintaining during the passage of said current ananode current density from 0.1'to 0.3 amchloride.

References Cited in the file of this patent UNITED STATES PATENTSWeintraub Feb. I, 1910 Pyk June 26, 1951 6 FOREIGN PATENTS 635,267 GreatBritain Apr. 5, 1950 81,510 Argentina Sept. 25, 1951 947,983 GermanyJuly 16, 1935 OTHER REFERENCES Chemical Abstracts, v01. 35 (1941), page3530, abstract of publication by Gratsianskii.

Transactions of The Electrochemical Society, vol. 87 (1945), pages551-567, article by Kroll.

1. METHOD FOR REFINING TITANIUM METAL WHICH COMPRISES PASSING ELECTRICCURRENT THROUGH AN ELECTROLYTIC CELL HAVING AN ANODE AND A CATHODE AND AMOLTEN HALIDE SALT ELECTROLYTE, EMPLOYING IMPURE TITANIUM METAL AS THEANODE IN SAID CELL, SAID ELECTROLYTE BEING COMPOSED OF AT LEAST ONECOMPOUND SELECTED FROM THE GROUP CONSISTING OF THE HALIDE SALTS OFALKALI METALS, ALKALINE EARTH METALS, AND MAGNESIUM, SAID ELECTROLYTEFURTHER CONTAINING SOLUBLE TITANIUM COMPOUNDS SELECTED FROM THE GROUPCONSISTING OF TITANIUM DICHLORIDE AND TITANIUM TRICHLORIDE INCONCENTRATION FROM 0.1 TO 5.0 MOLAL, SAID SOLUBLE TITANIUM COMPOUNDSCOMPRISING AT LEAST 40% TITANIUM DICHLORIDE, AND MAINTAINING DURING THEPASSAGE OF SAID CURRENT AN